This invention relates to a magnetic recording medium, and more specifically to a magnetic recording medium containing .alpha.-Al.sub.2 O.sub.3 particles in its magnetic layer.
In the manufacture of audio or video tapes, floppy disks, or other magnetic recording medium, it is a generally known practice to add some reinforcement to the magnetic layer so as to protect it from damage due to contact with magnetic heads. Particles of nonmagnetic inorganic material, such as Al.sub.2 O.sub.3, Cr.sub.2 O.sub.3, TiO.sub.2, SiO.sub.2, SiC, .alpha.-Fe.sub.2 O.sub.3, or ZnO.sub.2, are used for this purpose. An inorganic material of proper type, particle size, and proportion must be chosen; otherwise, damage or accelerated wear of the head, deterioration of the electromagnetic conversion characteristics, or other trouble can result. In recent years the demand for qualitative improvements of magnetic recording media, especially video tapes, has become severer than before. In order to meet the requirements for better picture quality, the adoption of microfine magnetic powders and highly dispersible binders, and smoothening of tape surface are under way. These measures are rather unfavorable, however, from the viewpoint of the magnetic coat durability or deposition on the head. For this reason greater durability and improved head cleaning effect are being called for. If the inorganic powders in common use are employed for such high picture quality video tapes, problems will arise including poor electromagnetic transfer characteristics, tape damage due to inadequate reinforcement, and insufficient head cleaning effect which, in turn, causes increased powder deposition on the head and inferior picture quality.
The present inventors have done intensive investigations with the hope of solving the problems associated with the improvements in the picture quality of video tapes through a proper choice of the type and particle size of inorganic material as a reinforcement of the magnetic coated film.
Among the existing nonmagnetic inorganic reinforcing materials, Cr.sub.2 O.sub.3, Al.sub.2 O.sub.3 and the like have been favorably evaluated. Nevertheless, they have not proved fully satisfactory when used directly as reinforcements for high picture quality magnetic recording media. The present inventors, in the hope of realizing the above object, studied Al.sub.2 O.sub.3 particles, one of the desirable inorganic reinforcements. It is generally known that Al.sub.2 O.sub.3 occurs stably in two crystalline forms, .alpha.-Al.sub.2 O.sub.3 and .gamma.-Al.sub.2 O.sub.3. The latter is finer in particle size and softer than the former. On the other hand, .alpha.-Al.sub.2 O.sub.3 is harder and coarser. For the reinforcement of magnetic recording media, either Al.sub.2 O.sub.3 alone or a mixture of both is offered.
.alpha.-Al.sub.2 O.sub.3, coarser and difficult to reduce the particle size, achieves a reinforcing effect, but is not a suitable reinforcing material for high picture quality magnetic tapes because of its danger of damaging the magnetic head and inferior electromagnetic transfer characteristics. .gamma.-Al.sub.2 O.sub.3 is also unsuitable because of low reinforcing effect and poor dispersion on account of the too fine particle size. In the Al.sub.2 O.sub.3 mixture of .alpha.- and .gamma.-types, therefore, the presence of .gamma.-Al.sub.2 O.sub.3 is of no appreciable merit.
It has not been found that an .gamma.-Al.sub.2 O.sub.3 produced by the organic alumina process and having an average particle size of 0.1 to 0.3 .mu.m as measured by electron microscopy and being angular or jagged in the shape of the particles is excellent in electromagnetic transfer characteristics, running durability, head cleaning effect, and other properties and is suitable for use as a reinforcing material for video tapes of high picture quality.
The present invention therefore aims at providing a magnetic recording medium excellent in electromagnetic transfer characteristics and endurance running properties with limited deposition on heads.